1. Field of the Invention
The present invention relates to an optical pickup device for reading or writing information to an optical disk having recording surfaces of a plurality of layers, and to an optical disk playback device provided therewith.
2. Description of the Related Art
In recent years, greater recording capacity for optical disks had been required, and DVD (Digital Versatile Disk) and other optical disks obtained by laminating the recording surfaces of a plurality of layers have therefore come into practical use. In an optical disk recording and playback device for recording and playing back data to and from an optical disk having the recording surfaces of such a plurality of layers, as shown in FIG. 5, while an optical pickup device 500 for reading or writing data to an optical disk D is moved in the focus-adjusting direction as indicated by the arrow, an object lens 4 focuses a laser beam on one of the recording surfaces of the layers of the optical disk D (layer 0 or layer 1), and data is read or written, while an optical pickup device 500 for reading or writing data to an optical disk D is moved in the focus-adjusting direction as indicated by the arrow shown in FIG. 5. Therefore, the movement of the optical pickup device 500 is essential and data cannot be read in a short period of time when reading a large quantity of data recorded across a plurality of layers. In a similar manner, data cannot be written in a short period of time when writing a large quantity of data that must be recorded across a plurality of layers. In view of the above, techniques are being studied in which the reading speed and the writing speed can be increased, and the time required to write data can be shortened, by increasing the rotational drive speed of the optical disk D. Nevertheless, there are naturally limits to increasing the rotational speed of the optical disk D due to the need for noise elimination and to drawbacks in the reading precision and writing precision of the optical pickup device 500.
In view of the above, known optical disk recording and playback devices (refer to Japanese Patent Application Laid-open Nos. 2000-235733, and 2000-298950, for example) are structured such that two different pickups respectively corresponding to the recording surfaces of two layers of an optical disk are provided and optical pickup is separately performed for each recording surface in order to shorten the time required to write data without depending on higher rotational drive speeds of the optical disk. With such optical disk recording and playback devices, information can be simultaneously read from and simultaneously written to the recording surfaces of two layers, and the time required to read and write a large quantity of data across two layers can be shortened.
There are also optical pickup devices (refer to Japanese Patent Application Laid-open No. HEI 8-96406, for example) in which defocusing aberration of a predetermined value or more and spherical aberration of a predetermined value or less are maintained and an increase in recording density is attempted by optimizing the distance and refractive index in transmission between the recording surfaces of the optical disk, the number of layers of the recording surfaces, the numerical aperture of the object lens in the optical pickup device, and the wavelength of a laser beam directed to the optical disk.
There are also optical pickup devices (refer to Japanese Patent Application Laid-open No. 2002-25098, for example) in which the leakage (crosstalk between the layers) of the laser beam between the recording surfaces of the plurality of layers is reduced and a reduction in the noise during the reading and writing of data is attempted by optimizing the size of the photodetector for detecting the laser beam reflected by the optical disk.